highslide-software/highcharts.com — pull request #3580 by paulo-raca (2014-11-11 14:20:34+00:00)
Title: Use 'crispEdges' option when drawing heatmaps to prevent the tiny gaps between cells.

The artifact used to be very visible in Chrome and sometimes visible in Firefox
E.g.: http://www.highcharts.com/maps/demo/heatmap